What is a mnemonic to quickly remember the five stages of lung development and their approximate gestational weeks?

Mnemonic for the Five Stages of Lung Development

Use "Every Person Can See Air" to remember the five stages of lung development: Embryonic (0-7 weeks), Pseudoglandular (5-17 weeks), Canalicular (17-26 weeks), Saccular (26 weeks-birth), and Alveolar (late gestation through first 1-3 years of life). 1, 2

The Five Stages with Gestational Timing

Embryonic Stage (0-7 weeks)

• The lung primordium forms as a diverticulum of the foregut during the first seven weeks of gestation 2, 3
• This represents the initial laying down of the lung structure from the primitive foregut 2

Pseudoglandular Stage (5-17 weeks)

• The lung resembles a tubulo-acinar gland with epithelial tubes sprouting and branching into surrounding mesenchyme 2, 3
• All prospective conducting airways are formed by the end of this stage, with airway branching completed by 16-18 weeks 2, 4
• The acinar limits become recognizable in the final week of this period 2

Canalicular Stage (17-26 weeks)

• Peripheral lung tubules widen and the cuboidal epithelium differentiates into type I and type II pneumocytes 1, 2
• The first thin air-blood barrier forms during this critical period 1, 2
• Surfactant production begins, marking a crucial milestone for potential viability 1, 2
• Clara cells producing CC10 appear by the equivalent of gestational day 14.5 in mouse models 1

Saccular Stage (26 weeks gestation to birth)

• The pulmonary parenchyma expands markedly with thinning of connective tissue between airspaces 1, 2
• The surfactant system undergoes further maturation 1, 2
• Type II pneumocytes expressing surfactant protein C become detectable 1
• Primitive double-capillary septa are present but not yet remodeled 2

Alveolar Stage (late gestation through first 1-3 years of life)

• True alveoli are generated through septation, dramatically increasing gas-exchange surface area 1, 2
• Primitive septa with double capillary networks remodel into mature single-layer capillary networks 1, 2
• This process requires angiogenesis to form the second capillary layer and continues postnatally 1
• At birth, the lung is functional but structurally immature, with alveoli practically missing 2

Clinical Relevance of Developmental Stages

The timing of developmental interruption directly determines the severity of pulmonary outcomes—earlier disruption results in more pronounced hypoplasia and poorer prognosis. 1

Critical Period Vulnerabilities

• Canalicular stage (17-26 weeks): Congenital diaphragmatic hernia occurring during this window mechanically compresses the developing lung, leading to pulmonary hypoplasia 1
• Saccular stage (26 weeks-birth): Very preterm infants born during this period are at risk for bronchopulmonary dysplasia (BPD), characterized by uniformly arrested alveolar development 1
• Alveolar stage disruption: Newborns and young infants lack collateral ventilation pathways (pores of Kohn, Lambert channels), predisposing them to atelectasis from airway obstruction 1

Practical Application

• Understanding these stages helps predict which preterm infants will benefit most from antenatal corticosteroids for lung maturation (recommended before 37 weeks gestation) 5, 6
• The saccular period (24-26 weeks) represents a critical window where air pollution exposure most significantly affects subsequent childhood asthma outcomes 5